A sensor such as a Hall sensor for use in the electric motor detects a change in magnetic field by a sensor magnet provided separately from a field magnet for generating a rotation torque. The position of a magnetic pole of the field magnet is detected and drive current is changed over based on an output from the sensor so as to rotate the rotor.
For example, technology of using a magnet holding ring in which a plurality of thin plate like magnets are provided at an equal interval along its periphery in a brushless DC motor has been disclosed.
Because such a sensor magnet is an isotropic magnet, lines of magnetic force pass through the inside of the magnet when magnetic force is produced and thus, magnetization up to near a boundary between N pole and S pole is difficult. Thus, improvement of the accuracy of detecting the position of the field magnet has a limit.